1. Field of the Invention
The present invention relates to the coating of fibers by CVD to make them more suitable for use in fiber-reinforced composites. More, specifically, the present invention concerns a method and an apparatus by which CVD coatings can be applied to fibers more efficiently and economically.
2. Description of Related Art
High-performance fibers are being increasingly used as the reinforcement of plastic, metal, ceramic, and carbon matrix composites. An important role of the fibers is to toughen the composite to prevent brittle failure. The degree of toughness attained is greatly affected by the bond strength between the fibers and matrix. If the bond strength is too high, the cracks propagate through the fibers; if too low, the load is not transferred to them. At an optimum bond strength, crack propagation is hindered by energy dissipation due to pullout.
The most demanding of these applications are those involving high operating temperatures. In such environments, the matrix may chemically react with, or dissolve the fiber. Although chemical reaction may in some cases be beneficial, it usually leads to drastic reductions in strength and toughness.
In many cases, problems encountered at high-temperatures can be solved by applying barrier coatings on the fibers, a favored technique being chemical vapor deposition (CVD). As the name implies, CVD involves the deposition of coatings onto substrates by chemical reaction from the vapor phase.
A typical process for applying barrier coatings onto fibrous substrates is to move the fibrous substrate through a CVD reactor in the presence of reagent gases, preferably while heating the gases. Difficulties encountered in the CVD technique of applying barrier coatings to fibrous substrates include:
(1) soot particles form by homogeneous nucleation and growth and many of them deposit on the reactor inner wall or liner inner wall, if a liner is used; and PA1 (2) most fibers, prior to coating, contain at least some broken filaments ("fuzz"); some filaments break during the coating process, and some of the fuzz becomes attached to the innermost surface of the reactor assembly by CVD and then break away from the moving fiber. PA1 a uniform inside diameter of sufficiently large size to receive a tool for periodic cleaning while the apparatus is in use, PA1 an unconstricted outlet, PA1 an ambient-atmosphere-excluding (preferably constricted) inlet through which the fibrous material enters, and PA1 intake means for receiving a gaseous mixture comprising one or more reagents and carrier gas (herein referred to as "gaseous mixture") that can coat the fibrous material by CVD. PA1 a reactor assembly having an inner surface defining an elongate through passageway, the elongate passageway having an axis and opposite axially spaced first and second ends; PA1 means for progressively moving the elongate inorganic fibrous substrate axially through the elongate passageway from the first end toward the second end; PA1 means for introducing into the passageway a gaseous mixture adapted to deposit a coating on the inorganic fibrous substrate; PA1 an elongate scraping member having a peripheral surface and extending axially of the passageway along one side of said inner surface with a longitudinally extending portion of the peripheral surface of the scraping member in superposed position relative to a longitudinal axially extending portion of the inner surface; and PA1 means for causing relative revolving movement between the inner surface and the elongate scraping member around the axis of the passageway to progressively change the longitudinal axially extending portion of the inner surface adjacent the scraping member and cause extraneous debris along the inner surface to be progressively scraped away by the scraping member. PA1 the inner surface is generally cylindrical; PA1 the reactor assembly comprises a fixed elongate outer tube having opposite ends and a through opening around the axis axially extending between the ends; PA1 an elongate liner tube having the inner surface, means for mounting the liner tube within the opening in the outer tube for rotational movement around the axis relative to the outer tube and to the scraping member; and PA1 means for rotating the liner tube around the axis. PA1 providing a reactor assembly having an inner surface defining an elongate through passageway, the elongate passageway having an axis and opposite axially spaced first and second ends; PA1 progressively moving the elongate inorganic fibrous substrate axially through the elongate passageway from the first end toward the second end; PA1 introducing into the passageway a gaseous mixture adapted to deposit a coating on the inorganic fibrous substrate; PA1 providing an elongate scraping member having a peripheral surface and extending axially of the passageway along one side of the inner surface with a longitudinally extending portion of the peripheral surface of the scraping member in superposed position relative to a longitudinal axially extending portion of the inner surface; and PA1 causing relative revolving movement between the inner surface and the elongate scraping member around the axis of the passageway to progressively change the longitudinal axially extending portion of the inner surface on which is superposed the scraping member to cause extraneous debris along the inner surface to be progressively scraped away by the scraping member.
Regarding the first problem, the rate of soot formation is slow if a low pressure CVD system is used. However, the rate is pronounced in atmospheric pressure CVD (APCVD) systems, and the rate increases with increasing partial pressure(s) of reagent(s). As economic considerations dictate high fiber throughput, high partial pressures are preferred.
As for the problem presented by broken filaments, the process of the fuzz becoming attached to the reactor inner wall by CVD and then breaking away from the moving fiber accelerates as the fuzz already attached to the reactor inner wall "catches" additional broken filaments entering the reactor. The "fuzzballs" (i.e., a combination of soot and fuzz) thus formed deplete reagents by serving as substrates for CVD. If not removed periodically or held to de minimis size, the moving fibrous substrate may rub against stationary fuzzballs and may even break.
In both subatmospheric (i.e. below atmospheric pressure) and atmospheric pressure CVD coating of fibers, fuzzballs are typically periodically manually removed.
The parent application (Ser. No. 07/383,923) provides an apparatus by which a coating can be continuously applied by APCVD to a multiplicity of filaments or fibers such as a tow of monofilaments or a yarn or a strip of woven fabric. Such a multiplicity of filaments or fibers is sometimes hereinafter referred to as "the fibrous material."
Briefly, the apparatus of the Ser. No. 07/383,923 application includes:
a furnace,
a straight, elongated furnace tube extending through the furnace, which furnace tube is formed with
The furnace tube can be fitted with an elongated hollow cylindrical liner that is easily replaced simply by pulling it through the outlet. In this way deposits are eliminated without disturbing either the inlet for the fibrous material or the intake means. The replacement is performed when deposits on the liner inner surface unduly constrict the open cross-sectional area of the liner. A tool may be periodically inserted into the reactor tube, when it is desired not to utilize a liner, or into the liner, to remove soot and fuzzballs. However, when the apparatus of the Ser. No. 07/383,923 application has been used to coat a ceramic tow or other fibrous material, it has been necessary to use a cleaning tool to remove fuzzballs and soot in some instances as often as every ten minutes.